We discovered an inherited immunodeficiency disease in certain hybrid families of chickens referred to as UCD 140. The chief characteristics of the disease are (1) multifactorial inheritance; (2) acquired (late onset) selective IgG deficiency; (3) increased IgM serum levels producing a dysgammaglobulinemia; (4) increased serum IgG in some birds after several months of age; and (5) manifestations of autoimmune pathology late in the disease. We propose (a) to determine whether the disease is caused by a major gene; (b) to correlate the age-dependent relationship of Ig levels, suppressor activity of peripheral blood mononuclear cells, surface and cytoplasmic Ig markers of PBL, development of immune responses, function of T and B cells, and onset of autoimmune manifestations.